The invention concerns circuitry for reversing a magnetic field.
Circuitry of this type is employed for example in magneto-optical recording and playback equipment to reverse the magnetism of the magnetic layer in magneto-optical software.
An example of known magneto-optical software is the magneto-optical disk, which has a magneto-optical layer beyond a light-permeable layer. Information can be entered into and recalled from the magneto-optical layer. How data are entered into a magneto-optical disk will now be described.
A laser beam is focused on the disk and heats the magneto-optical layer to a temperature in the vicinity of its Curie point. It is, however, usually sufficient to heat the layer only to approximately its compensation temperature, which is below the Curie temperature. An electromagnet is positioned beyond the focal point on the disk and magnetizes the area heated by the laser beam in one direction or the other. Since the heated area cools down to below the compensation temperature again once the laser beam is turned off, the magnetic orientation established by the electromagnet is retained. It freezes in, so to speak. The individual bits are accordingly stored in domains of different magnetic orientation. One orientation corresponds for example to a domain of logical ONE and the other to a domain of logical ZERO.
The Kerr effect is exploited to recall the information. The plane of polarization of a linearly polarized beam of light is rotated while being reflected in a magnetized mirror around a measurable angle. The plane of polarization of the reflected beam of light is rotated right or left in accordance with the magnetic orientation of the mirror. Since, however, the individual domains on the disk act like magnetized mirrors, the plane of polarization of a beam of light that scans the domains will be rotated right or left around a measurable angle in accordance with the magnetic orientation of the domain just scanned.
From the angle of rotation of the plane of polarization of the beam of light reflected from the disk, an optical pick-up can determine whether the bit that is present is a ONE or a ZERO.
One known way of magnetizing a magneto-optical layer in one direction or the other is provided by circuitry that acts like an electromagnet with a coil beyond the magneto-optical disk. The coil must be large enough to allow remagnetization of the total area covered by the optical pick-up. This area will, depending on the type of recording-and-playback equipment, be either a radial or circular-segmental strip extending from the edge to the center of the disk. Since the field strength must attain a minimum over the total strip in order for the strip to be remagnetized, the cross-section of the coil will be relatively extensive and its inductivity accordingly relatively high.
The coil in another known approach is secured to the optical pick-up. The coil can for example be wound around the pick-up's objective lens. Since the coil in this version can be advanced along with the optical pick-up along the data tracks on the surface of the magneto-optical disk by a tracking circuit, a less extensive cross-section and accordingly lower inductivity will suffice to generate the same minimum field strength, because it is no longer a radial or circular-segmental strip but only a small and circular for example area with the almost dimensionless laser spot at its center that needs to be remagnetized in the magneto-optical layer.